1. Field of the Invention
The present invention relates to an engraving machine for gravure printing, and more particularly, it relates to a gravure printing engraving machine in which a stylus mounted in an engraving head is controlled in accordance with a modified engraving signal to engrave the surface of a gravure cylinder.
2. Description of the Related Art
A gravure engraving machine is an apparatus in which a gravure cylinder having a copper plated surface, used in a gravure printing process, is selectively rotated and engraved so as to form numerous minute pyramid-shaped pits regularly arranged and referred to as "cells" in the surface of the cylinder. The amount of ink held in each cell and the number and orientation of a group of cells determines the size and appearance of subsequently printed matter. Therefore, the engraving of such cells in a gravure cylinder is extremely important. The size of each cell, in particular is of great importance because depth and width of each cell determines image density of the subsequently printed matter.
The engraving head on an engraving machine is used to form the cells in the surface of the cylinder. The engraving head includes a stylus having one end provided with a diamond bit. The stylus is vibrated at a variety of frequencies, generally in the kHz range, to perform the engraving.
The waveforms depicted in FIG. 8 represent portions of typical signals applied to the engraving head. The waveforms depicted are representative of the type waveforms necessary for displacement of the stylus. For instance, a complete engraving signal is typically obtained by superimposing a high-frequency carrier signal such as the waveform illustrated in FIG. 8(a) and a density signal (or image signal) such as the waveform illustrated in FIG. 8(b).
Applying the signal, an "engraving signal", formed by combining the waveforms in FIG. 8(a) and 8(b) to the engraving head, a cell corresponding in depth and width to the density signal can be engraved in the surface of the cylinder. A one-dot-dash line S in FIG. 8(c) corresponds with the surface of the cylinder while a region marked with hatched lines correspond to the cell to be engraved in the surface of the cylinder. The oscillating waveform in FIG. 8(c) corresponds to the combined "engraving signal" applied to the engraving head.
When the density signal mentioned above is applied to the engraving head, the displacement of the stylus becomes extraordinarily large in a position corresponding with a change in the density signal. Specifically, the displacement of the stylus overshoots its intended depth due to the change of the density signal, or rather at the point where the signal changes. FIG. 8(d) depicts the waveform sent to the engraving head with the displacement effected by the carrier signal omitted. A comparison of the combined signal depicted in FIG. 8(c) with the signal depicted in FIG. 8(d) shows that the desired displacement of the engraving head is not obtained.
Thus, when the engraving signal as shown in FIG. 8(c) is employed to drive the stylus, the displacement of the stylus assumes a waveform P as shown in FIG. 8(e) and causes a cell to be engraved with undesirable dimensions and position, and consequently this brings about degradation of quality in the resultant printing, including blur in an edge portion of an image.
In automatic gain control circuitry for example, a technology for suppressing overshoot in a signal is disclosed in Japanese Unexamined Patent Publication No. 29045/1974. However, this control circuit technology utilizes feedback of an output signal to suppress overshoot. However, such prior art technology employing feedback in a circuit cannot be applied to a mechanism of varying output, like the displacement of the stylus in the gravure printing engraving machine, since such output is very difficult to detect.